The present invention relates to a device for aiding persons having a speech handicap.
It is known that speech depends on at least two independent parameters. One of them determines the level of the sounds emitted and the other the phonetic content of these sounds. The mechanism which monitors the level or frequency of the sounds is called phonation, whilst the mechanism which determines the phonetic content of these sounds is called articulation. In the present specification, only the phonation mechanism will be considered.
The level of the human voice is determined by the larynx in the following manner. An expiration is first necessary to produce a sound. In opposition to the conditions of normal respiration, the glottis, that is to say, the space between the vocal cords, is closed or very greatly restricted. In this manner a higher pressure is produced in the thorax than during normal expiration. This pressure which is exerted on the vocal cords, has the effect of separating the cords from each other so that a flow of air is produced which, passing through the glottis, terminates in the space formed by the throat and mouth. However, the glottis narrows the path for the expired air so that, in this position, the speed of the air is appreciably higher than in the trachea. It follows that the air pressure is very low at the level of the glottis thus permitting the glottis to close again and the phenomena begins all over again.
In other words, the vocal cords vibrate sufficiently to allow the passage between them of puffs of air, the number per second of which represents the level or frequency of the sound emitted. This frequency depends mainly on the tension of the vocal cords and only then on the pressure of air under the glottis. It is clear that these two parameters can be modified by the muscles of the larynx and those of the thorax.
Some diseases of the larynx have an effect on the power of emitting sounds or speech. These are in particular those which concern the vocal cords such as paralysis thereof, the weakness of the muscles determining their tension, and cancer of the larynx which in the first place attacks the vocal cords.